Solid state image sensors fabricated from semiconductor materials are known. Some image sensors may be produced using CMOS processing technologies.
An image sensor comprises at least one sensor and associated circuitry for controlling the operation of the image sensor and for manipulation of the signals that are output from the image sensor.
An image sensor may comprise one or more pixels. Each pixel comprises a photodiode which may be formed as a doped region within a semiconductor substrate. The energy of incident photons removes electrons from the outer orbits of atoms within the photosensitive portion thus generating a charge. The pixel also comprises readout circuitry.
The photodiode makes use of an electric field at a P-N junction to cause the photo generated electron to move away from the ion and prevent re-combination and loss of the signal. However, these P-N junctions have a small leakage current which the photodiode cannot distinguish from a current which is generated by light. This leakage current is present in the dark, and so this leakage current is commonly known as dark current. The term “dark” is understood to be a condition where light is either absent, or where light that is incident on the image sensor does not cause the photo generation of charge by the photodiode. This can either be because the photodiode is shielded, or because it is held at a potential, for example a reset potential, that prevents the accumulation of charge at the photodiode.
This dark current may be a limiting factor in the performance characteristics of photodiodes. Dark current is temperature dependent. Dark current may also be dependent on impurities in the silicon of the photodiode. Dark current may vary with any non-uniformity in doping gradients across the die or non-uniformity introduced by etching variations.